1. Field of the Invention
The present invention relates to a light emitting device having a vertical structure and a method for manufacturing the same, and more particularly, to a light emitting device having a vertical structure and a method for manufacturing the same which are capable of damping impact generated in a substrate separation process, and achieving an improvement in mass productivity.
2. Discussion of the Related Art
Light emitting diodes (LEDs) are well known as a semiconductor light emitting device which converts current to light, to emit light. Since a red LED using GaAsP compound semiconductor was commercially available in 1962, it has been used, together with a GaP:N-based green LED, as a light source in electronic apparatuses, for image display.
The wavelength of light emitted from such an LED depends on the semiconductor material used to fabricate the LED. This is because the wavelength of the emitted light depends on the band gap of the semiconductor material representing energy difference between valence-band electrons and conduction-band electrons.
Gallium nitride (GaN) compound semiconductor has been highlighted. One of the reasons why GaN compound semiconductor has been highlighted is that it is possible to fabricate a semiconductor layer capable of emitting green, blue, or white light, using GaN in combination with other elements, for example, indium (In), aluminum (Al), etc.
Thus, it is possible to adjust the wavelength of light to be emitted, using GaN in combination with other appropriate elements. Accordingly, where GaN is used, it is possible to appropriately determine the materials of a desired LED in accordance with the characteristics of the apparatus to which the LED is applied. For example, it is possible to fabricate a blue LED useful for optical recording or a white LED to replace a glow lamp.
On the other hand, initially-developed green LEDs were fabricated using GaP. Since GaP is an indirect transition material causing a degradation in efficiency, the green LEDs fabricated using this material cannot practically produce light of pure green. By virtue of the recent success of growth of an InGaN thin film, however, it has been possible to fabricate a high-luminescent green LED.
By virtue of the above-mentioned advantages and other advantages of GaN-based LEDs, the GaN-based LED market has rapidly grown. Also, techniques associated with GaN-based electro-optic devices have rapidly developed since the GaN-based LEDs became commercially available in 1994.
GaN-based LEDs have been developed to exhibit light emission efficiency superior over that of glow lamps. Currently, the efficiency of GaN-based LEDs is substantially equal to that of fluorescent lamps. Thus, it is expected that the GaN-based LED market will grow significantly.
Despite the rapid advancement in technologies of GaN-based semiconductor devices, the fabrication of GaN-based devices suffers from a great disadvantage of high-production costs. This disadvantage is closely related to difficulties associated with growing of a GaN thin film (epitaxial layer) and subsequent cutting of finished GaN-based devices.
Such a GaN-based device is generally fabricated on a sapphire (Al2O3) substrate. This is because a sapphire wafer is commercially available in a size suited for the mass production of GaN-based devices, supports GaN epitaxial growth with a relatively high quality, and exhibits a high processability in a wide range of temperatures.
Further, sapphire is chemically and thermally stable, and has a high-melting point enabling implementation of a high-temperature manufacturing process. Also, sapphire has a high bonding energy (122.4 Kcal/mole) and a high dielectric constant. In terms of a chemical structure, the sapphire is a crystalline aluminum oxide (Al2O3).
Meanwhile, since sapphire is an insulating material, available LED devices manufactured using a sapphire substrate (or other insulating substrates) are practically limited to a lateral or vertical structure.
In the lateral structure, all metal contacts for use in injection of electric current into LEDs are positioned on the top surface of the device structure (or on the same substrate surface). On the other hand, in the vertical structure, one metal contact is positioned on the top surface, and the other contact is positioned on the bottom surface of the device structure after removal of the sapphire (insulating) substrate.
In addition, a flip chip bonding method has also been widely employed. In accordance with the flip chip bonding method, an LED chip, which has been separately prepared, is attached to a sub-mount of, for example, a silicon wafer or ceramic substrate having an excellent thermal conductivity, under the condition in which the LED chip is inverted.
However, the lateral structure or the flip chip method suffers from the problems associated with poor heat release efficiency because the sapphire substrate has a heat conductivity of about 27 W/mK, thus leading to a very high heat resistance. Furthermore, the flip chip method has also disadvantages of requiring large numbers of photolithography process steps, thus resulting in complicated manufacturing processes.
To this end, LED devices having a vertical structure have been highlighted in that the vertical structure involves removal of the sapphire substrate.
In the fabrication of such a vertical LED structure, a laser lift off (LLO) method is used to remove the sapphire substrate, and thus, to solve the problems caused by the sapphire substrate.
However, it is impossible to completely remove the sapphire substrate at once, using the LLO method, due to the size and limited uniformity of a laser beam used in the LLO method. For this reason, uniform small-size laser beams are irradiated to respective portions of the sapphire substrate, in order to the entire portion of the sapphire substrate.
In the LLO method, stress is applied to the GaN thin film upon incidence of a laser beam. In order to separate a sapphire substrate 1 and a GaN thin film 2 from each other, as shown in FIG. 1, it is necessary to use a laser beam having a high energy density. The laser beam resolves GaN into a metal element, namely, Ga, and nitrogen gas (N2).
The resolved nitrogen gas exhibits a high expansion force, so that it applies considerable impact not only to the GaN thin film 2, but also to a support layer for the GaN thin film 2 and metal layers required for the fabrication of the device. As a result, a degradation in bondability occurs primarily. In addition, a degradation in electrical characteristics occurs.
Such results can be observed from FIG. 2. As shown in FIG. 2, wave patterns exhibited as having irregularities may be formed at the peripheral portion of the GaN thin film after completion of the LLO process. Also, during the LLO process, many poor bonding portions may be observed on the thin film.
Furthermore, the above-mentioned method incurs damage of a back surface of the GaN thin film, on which an LED is to be formed, in a region where laser beams overlap with each other. There may also be a phenomenon that cracks formed at poor-quality portions of the GaN thin film are propagated to other portions of the GaN thin film.
In order to prevent such a phenomenon, various methods have been used. For example, in one method, the GaN thin film is etched in certain regions, to separate respective devices from one another, as shown in FIG. 3. Thereafter, a semiconductor wafer 5 made of Si, GaAs, etc. is bonded to the GaN thin film. In another method, a metal support 7 is formed on the GaN thin film in accordance with a plating method using a metal such as Cu, Au, or Ni, and the sapphire substrate 1 is then separated, as shown in FIG. 4.
In detail, in the case of FIG. 3, the bonding of the semiconductor wafer 5, which has a thermal expansion coefficient considerably different from that of a GaN material, is achieved by a bonding material 4 bonded to an electrode layer 3. For this reason, the wafer 5 may be greatly bent after the bonding process. In addition, a plurality of empty spaces causing formation of poor bonding interfaces may be formed.
In the above case, air may remain in the empty space of a trench defined between adjacent devices after the bonding process. This air is expanded by high thermal energy of a laser, thereby causing cracks to be formed at the GaN thin film 2 in a region around the trench.
On the other hand, in the case of FIG. 4, in which a support substrate is fabricated using a metal support 7 formed by plating of a metal such as Cu, Au, or Ni, there are advantages of a high thermal stability and reduced bending of the metal support 7 after the plating process, as compared to the semiconductor wafer bonding method.
In the case using the plating process, however, the bondability between laminated layers, for example, the GaN thin film 2, electrode layer 3, and coupling metal layer 6, may be degraded due to stress caused by the laser. Furthermore, there are adverse affects on the electrical characteristics of the metal layers 3 and 6 used for the fabrication of the GaN-based devices. In addition, there is a degradation in electrical characteristics associated with the electrical electrode material of the GaN thin film 2, namely, ohmic metal.
Due-to the above-mentioned reasons, it is necessary to minimize irradiation of laser beams in the substrate separation process using the laser beams, and impact caused by expansion of nitrogen gas generated during the laser beam irradiation.
Meanwhile, in the above-mentioned vertical LED structure, a trench etching is carried out after formation of the GaN thin film 2 on the sapphire substrate 1, namely, formation of a GaN LED structure, in order to trenches 8 for defining regions corresponding to respective devices.
The trench etching is continued until the sapphire substrate 1 is exposed. This trench etching is a prolonged process in that the etching depth thereof reaches about 5 to 10 μm.